High-rigidity oxymethylene polymer resin molding

ABSTRACT

An oxymethylene polymer resin shaped article having high flexural modulus, which is produced by molding an oxymethylene polymer resin, wherein the oxymethylene polymer resin is selected from the group consisting of an oxymethylene homopolymer resin, an oxymethylene copolymer resin and a mixture thereof. The oxymethylene copolymer resin comprises a copolymer of an oxymethylene monomer and a comonomer copolymerizable therewith, wherein the amount of the comonomer is extremely limited. The shaped article has the following characteristics: (1) a crystallinity of 72% or more; (2) an average crystallite size of 150 Å or more; (3) 70% by volume or more of the whole volume of the shaped article being comprised of spherulites each having a diameter of 60 μm or less; and (4) a thickness of 1 mm or more. The oxymethylene polymer resin shaped article of the present invention has an improved crystal structure, and exhibits excellent mechanical properties including especially high flexural modulus even when containing very little or no reinforcement. The oxymethylene polymer resin shaped article of the present invention can be advantageously used not only as a material for a sliding part and a precision part, such as a gear, a bearing and a lever, but also as a material for electric and electronic parts, such as a connector, a socket and a switch.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/JP97/04860 which has an Internationalfiling date of Dec. 25, 1997 which designated the United States ofAmerica.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an oxymethylene polymer resin shapedarticle having high flexural modulus. More particularly, the presentinvention is concerned with an oxymethylene polymer resin shaped articlewhich is produced by molding an oxymethylene polymer resin, wherein theoxymethylene polymer resin is selected from the group consisting of anoxymethylene homopolymer resin, an oxymethylene copolymer resin and amixture thereof. The oxymethylene copolymer resin comprises a copolymerof an oxymethylene monomer and a comonomer copolymerizable therewith,wherein the amount of the comonomer is extremely limited. The shapedarticle has an improved crystal structure, wherein the crystallinity is72% or more, the average crystallite size is 150 Å or more, and 70% byvolume or more of the whole volume of the shaped article is comprised ofspherulites each having a diameter of 60 μm or less. The shaped articlealso has a thickness of 1 mm or more. The oxymethylene polymer resinshaped article of the present invention exhibits excellent mechanicalproperties including especially high flexural modulus.

The oxymethylene polymer resin shaped article of the present inventioncan be advantageously used as a material for a sliding part and aprecision part. Further, the oxymethylene polymer resin shaped articleof the present invention exhibits high flexural modulus and high heatconductivity both at high temperatures, and, hence, can beadvantageously used in the field of parts for electric and electronicapparatuses.

2. Prior Art

A shaped article of an oxymethylene polymer resin has various excellentproperties, such as excellent mechanical properties, excellent fatigueresistance, excellent self-lubricating properties, and excellent heatresistance. Therefore, oxymethylene polymer resin shaped articles havebeen used in a wide variety of commercial applications, such as variousparts for use in various machines, for example, automobiles, electricapparatuses, electronic apparatuses and office automation machines.

These commercial applications include those applications in which anoxymethylene polymer resin shaped article is required to exhibitespecially high mechanical properties while maintaining the otherexcellent properties inherent in an oxymethylene polymer resin shapedarticle. In such applications, the mechanical properties of anoxymethylene polymer resin shaped article are improved by a method inwhich, before the molding of an oxymethylene polymer resin, a largeamount of a reinforcement (filler), such as a glass fiber or a carbonfiber, is incorporated into the oxymethylene polymer resin. However,with respect to the resultant composite shaped article havingincorporated thereinto a large amount of a reinforcement, there havebeen problems in that the shaped article has a large specific gravity, apoor sliding property and a poor surface appearance.

Therefore, various attempts have been made to improve the mechanicalproperties of the oxymethylene polymer resin shaped article by a methodusing very little or no reinforcement. Among these attempts, there havebeen attempts to achieve the above object by improving the crystalstructure of the oxymethylene polymer resin. As examples of knownmethods for improving the crystal structure of the oxymethylene polymerresin in an attempt to obtain a shaped article having improvedmechanical properties, there can be mentioned a method in which anoxymethylene polymer resin to be molded is subjected to isothermalcrystallization at a relatively high temperature, and a method in whichan oxymethylene polymer resin to be molded is subjected to annealing(see, for example, J. of Appl. Polym. Sci. vol. 1, p. 169 (1959); J.Macromol. Sci. Phys. B13(3), p. 323 (1977); Polymer vol. 19, p. 1163(1978); and J. of Appl. Polym. Sci. vol. 55, p. 489 (1995)). Theoxymethylene polymer resin shaped articles obtained by these methods areremarkably improved in crystallinity; however, these shaped articles aredisadvantageous not only in that the degree of the improvement in theflexural modulus is small, but also in that the extensibility andtoughness are reduced. Therefore, these oxymethylene polymer resinshaped articles cannot satisfactorily meet the needs of the commercialapplications.

As further examples of known methods for improving the crystal structureof the oxymethylene polymer resin so as to improve the mechanicalproperties of the shaped article obtained therefrom, there can bementioned a method in which a crystal nucleating agent is added to anoxymethylene polymer resin to be molded, and a method which utilizes aphenomenon such that when an oxymethylene polymer resin is heated to atemperature slightly higher than the melting temperature of the polymerresin, a non-melted crystal nucleus remains in the polymer resin andserves as a crystal nucleating agent. This facilitates thecrystallization of the polymer resin, wherein the non-melted crystalnucleus is presumed to be a portion still having a slight regularity instructure or having an unknown specific structure. Hereinafter, thisphenomenon is frequently referred to simply as "self-nucleatingphenomenon" (see, for example, Polymer vol. 20, p. 1470 (1979); andPolymer Bulletin 24, p. 445 (1990)). In the oxymethylene polymer resinshaped articles obtained by these methods, the formation of spheruliteshaving a very small diameter can be achieved. However, by these methods,it is difficult to obtain shaped articles having a remarkably improvedflexural modulus. Therefore, the oxymethylene polymer resin shapedarticles obtained by these methods are not always satisfactory.

Further, Polymer vol. 29, p. 793 (1988) discloses an oxymethylenehomopolymer resin shaped article in which both a formation ofspherulites having a very small diameter and an increase in thecrystallinity have been achieved by utilizing the self-nucleatingphenomenon. However, the flexural modulus of the shaped articledisclosed in this prior art document is only 3.54 GPa. That is, thisshaped article is not satisfactory with respect to the improvement inflexural modulus. From the molding conditions described in this priorart document, the present inventors have presumed that the reason whythe oxymethylene homopolymer resin shaped article disclosed in thisprior art document is not satisfactory with respect to the improvementin flexural modulus is because the growth of the crystal structure isunsatisfactory, especially the crystal structure which is evaluated interms of an average crystallite size. As a result, a satisfactorilylarge average crystallite size cannot be obtained.

SUMMARY OF THE INVENTION

In this situation, the present inventors have made extensive andintensive studies with a view toward developing an oxymethylene polymerresin shaped article which exhibits excellent mechanical properties evenwhen containing very little or no reinforcement (filler). As a result,it has unexpectedly been found that when an oxymethylene polymer resinshaped article has both a high degree of crystallization (specifically,characterized in that the crystallinity is 72% or more and the averagecrystallite size is 150 Å or more) and a high volume ratio of very smalldiameter spherulites (specifically, characterized in that 70% by volumeor more of the whole volume of the shaped article is comprised ofspherulites each having a diameter of 60 μm or less) and also has athickness of 1 mm or more, the shaped article exhibits excellentmechanical properties including especially high flexural modulus. Thepresent invention has been completed, based on the above finding.

Accordingly, it is a primary object of the present invention to providean oxymethylene polymer resin shaped article exhibiting excellentmechanical properties including especially high flexural modulus evenwhen containing very little or no reinforcement (filler).

The foregoing and other objects, features and advantages of the presentinvention will be apparent from the following detailed description takenin connection with the accompanying drawing and the appended claims.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1. is a flow chart showing how to improve the crystal structure ofan oxymethylene polymer resin (i.e., achieving both a high volume ratioof very small diameter spherulites and a high degree of crystallization)in the production process for the oxymethylene polymer resin shapedarticle of the present invention. In the production process shown inFIG. 1, the addition of a crystal nucleating agent to the oxymethylenepolymer resin and the use of improved conditions for the plasticizationof the oxymethylene polymer resin may be performed in combination.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, there is provided an oxymethylenepolymer resin shaped article having high flexural modulus, which isproduced by molding an oxymethylene polymer resin. The oxymethylenepolymer resin is selected from the group consisting of an oxymethylenehomopolymer resin, an oxymethylene copolymer resin and a mixturethereof. The oxymethylene copolymer resin comprises polymer chainscomprised of oxymethylene monomer units and oxyalkylene comonomer units.The oxyalkylene comonomer units are randomly present in the polymerchains collectively in an amount of from 0.01 to 1.0 mole, relative to100 moles of the oxymethylene monomer units.

The shaped article has the following characteristics (1) to (4):

(1) a crystallinity of 72% or more;

(2) an average crystallite size of 150 Å or more;

(3) 70% by volume or more of the whole volume of the shaped articlebeing comprised of spherulites each having a diameter of 60 μm or less;and

(4) a thickness of 1 mm or more.

For an easy understanding of the present invention, the essentialfeatures and various preferred embodiments of the present invention areenumerated below.

1. An oxymethylene polymer resin shaped article having high flexuralmodulus, which is produced by molding an oxymethylene polymer resin, theoxymethylene polymer resin being selected from the group consisting ofan oxymethylene homopolymer resin, an oxymethylene copolymer resin and amixture thereof, the oxymethylene copolymer resin comprising polymerchains comprised of oxymethylene monomer units and oxyalkylene comonomerunits, wherein the oxyalkylene comonomer units are randomly present inthe polymer chains collectively in an amount of from 0.01 to 1.0 mole,relative to 100 moles of the oxymethylene monomer units,

the shaped article having the following characteristics (1) to (4):

(1) a crystallinity of 72% or more;

(2) an average crystallite size of 150 Å or more;

(3) 70% by volume or more of the whole volume of the shaped articlebeing comprised of spherulites each having a diameter of 60 μm or less;and

(4) a thickness of 1 mm or more.

2. The shaped article according to item 1 above, wherein the molding isperformed by injection or extrusion.

3. The shaped article according to item 1 or 2 above, wherein, in thecharacteristic (3), 70% by volume or more of the whole volume of theshaped article is comprised of spherulites each having a diameter of 30μm or less.

4. The shaped article according to any one of items 1 to 3 above,wherein the oxymethylene polymer resin contains a crystal nucleatingagent in an amount of from 1 ppm by weight to 5% by weight.

5. The shaped article according to item 4 above, wherein the crystalnucleating agent is at least one compound selected from the groupconsisting of boron nitride, talc, silica, mica and carbon black.

6. The shaped article according to any one of items 1 to 5 above,wherein, prior to the molding, the oxymethylene polymer resin isplasticized at a temperature in the range of from the meltingtemperature of the resin to 10° C. above the melting temperature.

7. The shaped article according to any one of items 1 to 6 above,wherein, prior to the molding, the oxymethylene polymer resin has notbeen heated to a temperature that is 15° C. or more higher than themelting temperature of the resin.

8. The shaped article according to any one of items 1 to 7 above,wherein the oxymethylene polymer resin is crystallized at a temperaturein the range of from 30° C., below the melting temperature of the resinto the melting temperature during or after the molding.

9. The shaped article according to any one of items 1 to 8 above,wherein the oxymethylene polymer resin is annealed at a temperature inthe range of from 10° C. below the melting temperature of the resin tothe melting temperature after the molding.

10. The shaped article according to any one of items 1 to 9 above,wherein the oxymethylene polymer resin is an oxymethylene homopolymerresin.

11. The shaped article according to item 10 above, which has a flexuralmodulus of 3.8 GPa or more.

12. The shaped article according to any one of items 1 to 11 above,which is a part for an office automation machine, a part for an electricapparatus, a part for an electronic apparatus or a part for anautomobile.

The oxymethylene polymer resin shaped article of the present inventionis produced by injection molding or extrusion molding of an oxymethylenepolymer resin, and has the following characteristics (1) to (4):

(1) a crystallinity of 72% or more;

(2) an average crystallite size of 150 Å or more;

(3) 70% by volume or more of the whole volume of the shaped articlebeing comprised of spherulites each having a diameter of 60 μm or less;and

(4) a thickness of 1 mm or more.

The oxymethylene polymer resin used for producing the shaped article ofthe present invention may be any of an oxymethylene homopolymer resin,an oxymethylene copolymer resin or a mixture thereof. With respect tothe degree of polymerization of the oxymethylene polymer resin used inthe present invention, there is no particular limitation as long as theoxymethylene polymer resin can be used for molding by a conventionalmolding method. For example, an oxymethylene polymer resin having a meltindex (MI) of 1 to 50 g/10 min can be preferably used. The oxymethylenehomopolymer used in the present invention is a homopolymer comprisedessentially of oxymethylene monomer units (--CH₂ O--), which is obtainedby polymerizing formaldehyde or a cyclic oligomer of formaldehyde, suchas a formaldehyde trimer (trioxane) or a formaldehyde tetramern(tetraoxane).

On the other hand, the oxymethylene copolymer resin used in the presentinvention comprises polymer chains comprised of oxymethylene monomerunits and oxyalkylene comonomer units, and the oxyalkylene comonomerunits are randomly present in the polymer chains collectively in anamount of from 0.01 to 1.0 mole, preferably 0.01 to 0.8 mole, relativeto 100 moles of the oxymethylene monomer units. Examples of oxyalkylenecomonomer units include oxyethylene comonomer units, linear or branchedoxypropylene comonomer units, linear or branched oxybutylene comonomerunits, and oxyphenylethylene comonomer units. These comonomer units maybe used individually or in combination. Among these comonomer units,from the viewpoint of facilitating the improvement of the mechanicalproperties of the oxymethylene polymer resin shaped article, especiallypreferred are oxyethylene comonomer units, linear or branchedoxypropylene comonomer units and linear and branched oxybutylenecomonomer units.

The oxymethylene copolymer resin used in the present invention can beobtained by copolymerizing formaldehyde or a cyclic oligomer offormaldehyde, such as trioxane or tetraoxane, with a cyclic ether, suchas ethylene oxide, propylene oxide, epichlorohydrin, 1,3-dioxolane,glycol formal or diglycol formal. In the present invention, the aboveoxymethylene polymer resins may be used individually or in combination.

The oxymethylene polymer resin shaped article of the present inventionhas an improved crystal structure. Specifically, in the oxymethylenepolymer resin shaped article of the present invention, both a highdegree of crystallization (i.e., increase in both the crystallinity andthe average crystallite size) and a high volume ratio of very smalldiameter spherulites have been achieved.

The crystallinity of the oxymethylene polymer resin shaped article canbe evaluated by various analytical methods, such as specific gravitymeasurement, X-ray diffraction analysis, infrared (IR) absorptionspectroscopy and differential scanning calorimetry. Among these methods,X-ray diffraction analysis allows the crystallinity of the oxymethylenepolymer resin shaped particle to be easily determined.

A conventional oxymethylene polymer resin shaped article generally has acrystallinity of about 55 to 70%. On the other hand, in the oxymethylenepolymer resin shaped article of the present invention, it is requiredthat the crystallinity be 72% or more. In the shaped article of thepresent invention, the crystallinity is preferably 75% or more, morepreferably 78% or more. In the present invention, with respect to theupper limit of the range of the crystallinity of the oxymethylenepolymer resin shaped article, there is no particular limitation;however, the crystallinity is generally not higher than, for example,95%.

Explanation is made below on the average crystallite size of theoxymethylene polymer resin shaped article.

Generally, the crystal of a polymer is comprised of microcrystals, eachof which is called a "crystallite" and is considered to be a singlecrystal. As a method for quantitatively determining an averagecrystallite size, the Sherrer method using X-ray diffraction analysis iswidely employed. When the oxymethylene polymer resin in the presentinvention is subjected to X-ray diffraction analysis, an X-raydiffraction pattern having a peak at a diffraction angle (2 θ) of around34.6° is obtained, wherein the peak is ascribed to the (105) crystalface diffraction of the oxymethylene polymer resin. With respect to theobtained X-ray diffraction pattern, the value of a half width of thepeak at a diffraction angle (2 θ) of around 34.6° is obtained, and theobtained value of the half width is applied to the Sherrer's equation,thereby determining the crystallite size of the oxymethylene polymerresin. A conventional oxymethylene polymer resin shaped articlegenerally has an average crystallite size of about 80 to 120 Å. On theother hand, in the oxymethylene polymer resin shaped article of thepresent invention, it is required that the average crystallite size be150 Å or more. In the shaped article of the present invention, theaverage crystallite size is preferably 160 Åor more, more preferably 180Å or more. In the present invention, with respect to the upper limit ofthe range of the average crystallite size of the oxymethylene polymerresin shaped article, there is no particular limitation; however, theaverage crystallite size is generally not higher than, for example, 300Å.

In addition to the above-described requirements on the crystallinity andthe average crystallite size, when the oxymethylene polymer resin shapedarticle also satisfies both the below-described requirements on thevolume ratio of very small diameter spherulites and on the thickness ofthe shaped article, the shaped article can exhibit largely improvedmechanical properties.

In a conventional oxymethylene polymer resin shaped article, generally,the diameter of each spherulite is about 100 to 300 μm. On the otherhand, the oxymethylene polymer resin shaped article of the presentinvention has characteristics that 70% by volume or more of the wholevolume of the shaped article is comprised of spherulites each having adiameter of 60 μm or less, preferably 30 μm or less, more preferably 15μm or less. With respect to the lower limit of the range of the diameterof each of the above spherulites which constitute 70% by volume or moreof the whole volume of the shaped article, there is no particularlimitation; however, the diameter of each of such spherulites isgenerally not smaller than, for example, 0.5 to 1 μm.

Hereinbelow, a method for producing the oxymethylene polymer resinshaped article of the present invention is described.

Generally, an oxymethylene polymer resin shaped article is producedthrough the five steps mentioned at the bottom of the flow chart of FIG.1 (i.e., a polymer production step, a pretreatment step (pulyerization,addition of an additive, and the like), a plasticization step, a moldingstep, and an aftertreatment step).

The shaped article of the present invention can be produced by a methodwherein, in a production process comprising the above-mentioned stepsfor producing an oxymethylene polymer resin shaped article, thefollowing two operations for improving the crystal structure areperformed: an operation for achieving a high volume ratio of very smalldiameter spherulites and an operation for achieving a high degree ofcrystallization (specifically, for increasing both the crystallinity andthe average crystallite size).

Hereinbelow, the operation for achieving a high degree ofcrystallization (specifically, for increasing both the crystallinity andthe average crystallite size) is first explained, and next, theoperation for achieving a high volume ratio of very small diameterspherulites is explained.

Preferred examples of methods for achieving a high degree ofcrystallization (specifically, for increasing both the crystallinity andthe average crystallite size) include the following two methods: amethod in which the conditions under which the oxymethylene polymerresin is crystallized (hereinafter, frequently referred to simply as"crystallization conditions") are improved and a method in which theoxymethylene polymer resin is annealed after the molding. These twomethods may be employed individually or in combination.

First, explanation is made below on the method in which thecrystallization conditions for the oxymethylene polymer resin areimproved.

Generally, for increasing the degree of crystallization of a polymerresin, it is effective to crystallize the polymer resin at a temperaturejust below a temperature which adversely affects the polymer resin (forexample, just below a temperature which causes a heat decomposition ofthe polymer resin). However, when a polymer resin is crystallized duringa conventional molding method, the crystallization temperature of thepolymer resin is caused to be far lower than the melting temperature ofthe polymer resin. Specifically, for example, an oxymethylenehomopolymer resin has a melting temperature of 176° C. On the otherhand, in a conventional injection molding method, the temperature of amold is adjusted to 100° C., or less, which is far lower than themelting temperature of the resin (i.e., 176° C.). The employment of suchlow temperature as the mold temperature is intended to reduce themolding cycle time. When the oxymethylene homopolymer resin is molded bysuch conventional injection molding method, the large difference intemperature between the mold and the molten resin injected thereintocauses the cooling rate of the injected resin in the mold to becomeextremely high. As a result, the injected resin in the mold sufferssupercooling to a high extent. As a further result, the crystallizationof the resin occurs only at a temperature as low as 145° C. or less, sothat the high degree of crystallization required in the presentinvention cannot be achieved. The temperature range over which a resinbeing cooled undergoes supercooling varies depending on the cooling ratewhich the resin experiences when the resin is processed forcrystallization. That is, the lower the rate of cooling the resin, thesmaller the supercooling of the resin (i.e., the narrower thetemperature range over which the supercooling occurs). Thus, the lowerthe rate of cooling the resin, the higher the temperature at which thecrystallization of the resin occurs and, hence, the more easily thecrystallization occurs. However, it should be noted that, when a resinis exposed to high temperatures for a long time, problems may arisewherein the resin is adversely affected by heat (for example, the resinsuffers heat decomposition). That is, there is a dilemma such that it isdifficult to achieve a high degree of crystallization for a polymerresin without adversely affecting the polymer resin. The presentinventors have solved this dilemma by developing a preferredcrystallization method using improved conditions for the crystallizationof the polymer resin. Specifically, in the present invention, a highdegree of crystallization of the oxymethylene polymer resin is achievedby a method in which the crystallization temperature for a plasticizedoxymethylene polymer resin is adjusted to a relatively high temperature,namely, a temperature in the range of preferably from 30° C. below themelting temperature of the resin to the melting temperature, morepreferably from 27° C. below the melting temperature of the resin to themelting temperature, still more preferably from 25° C. below the meltingtemperature of the resin to the melting temperature. The method fordetermining the melting temperature of the oxymethylene polymer resinemployed in the present invention will be explained below.

As specific examples of methods for adjusting the crystallizationtemperature to the above specific preferred range in order to achieve ahigh degree of crystallization of the oxymethylene polymer resin, therecan be mentioned a method in which the rate of cooling a plasticizedresin introduced into a mold is adjusted by controlling the temperatureof the mold and a method in which a plasticized resin introduced into amold is subjected to isothermal crystallization. By using the abovemethods for adjusting the crystallization temperature, the supercoolingof the resin is suppressed to a minimum, so that a high degree ofcrystallization of the resin can be achieved. When the rate of coolingthe resin in the mold is adjusted by controlling the temperature of themold, the cooling rate is generally adjusted to 50° C./min or less,preferably 20° C./min or less, more preferably 10° C./min or less.

When the crystallization of a plasticized resin introduced into a moldis effected by isothermal crystallization, in order to achievecrystallization of most of the resin, the crystallization (during themolding) is generally performed by a method in which the mold ismaintained in a closed state and at a temperature in the above-mentionedrange for 30 seconds or more, preferably 60 seconds to 10 hours. Fromthe viewpoint of achieving a high degree of crystallization so as toobtain a shaped article having improved mechanical properties, it ispreferred that the crystallization of the resin is effected at atemperature as high as possible for a time as long as possible, providedthat the temperature and the time are selected so as not to adverselyaffect the resin (for example, so as not to cause heat decompositionthereof). In this respect, a more preferred crystallization temperaturefor the oxymethylene polymer resin is a temperature in the range of from27° C. below the melting temperature of the resin to 5° C. below themelting temperature of the resin.

In the present invention, the crystallization temperature for theoxymethylene polymer resin can be any temperature within the range offrom 30° C. below the melting temperature of the oxymethylene polymerresin to the melting temperature. For achieving a high degree ofcrystallization for the oxymethylene polymer resin, it is effective toperform the molding (during which the crystallization is effected) by astepwise cooling type molding method in which the resin introduced intothe mold is first gradually cooled to a holding temperature at which theresin can be maintained for a predetermined period of time withoutsuffering adverse effects (for example, heat decomposition), and thenthe resin is maintained at the holding temperature for the predeterminedperiod of time, followed by a further cooling of the resin. By thestepwise cooling type molding method (in which the resin is graduallycooled to a predetermined holding temperature and, then, the resin ismaintained at the holding temperature for a predetermined period oftime), the same effects as those obtained by annealing (optionallyeffected after the crystallization) can be obtained simultaneously withthe crystallization. (Annealing is described below in detail.)Therefore, the stepwise cooling type molding method described above isvery effective for achieving a high degree of crystallization for theoxymethylene polymer resin.

Then, as another method for achieving a high degree of crystallization(specifically, for increasing both the crystallinity and the averagecrystallite size), a method in which the oxymethylene polymer resin isannealed after the molding is explained below. In this method, theoxymethylene polymer resin shaped article obtained by an ordinarymolding method (in which the improved crystallization conditionsdescribed above may be utilized or may not be utilized) is annealed at avery high temperature, specifically a temperature in the range of from10° C. below the melting temperature of the oxymethylene polymer resinto the melting temperature, thereby achieving a high degree ofcrystallization. The "ordinary molding method" means a known moldingmethod which is generally used for the molding of a thermoplastic resin,such as injection molding, extrusion molding, press molding (compressionmolding), hot pressing molding or stamping molding, wherein moldingconditions generally employed in the art can be used. Of these moldingmethods, injection molding and extrusion molding are preferred. Withrespect to the conditions for the annealing of the oxymethylene polymerresin shaped article, for achieving a high degree of crystallization, itis preferred that the annealing temperature is as high as possible andthe annealing time is as long as possible, provided that the annealingtemperature and time are selected so as not to adversely affect theresin (for example, so as not to cause heat decomposition). From thispoint of view, it is preferred that the temperature for the annealingtreatment is in the range of from 8° C., below the melting temperatureof the oxymethylene polymer resin to the melting temperature and thatthe annealing time is in the range of from 30 seconds to 10 hours.

Next, a method for achieving a high volume ratio of very small diameterspherulites in producing the oxymethylene polymer resin shaped articleis explained below.

As preferred examples of methods for achieving a high volume ratio ofvery small diameter spherulites in producing the oxymethylene polymerresin shaped article, there can be mentioned the following two methods:a method in which a crystal nucleating agent is added to an oxymethylenepolymer resin to be molded and a method which utilizes improvedconditions for the plasticization of an oxymethylene polymer resin inorder to cause the self-nucleating phenomenon described above (i.e., aphenomenon such that a non-melted crystal nucleus remaining in aplasticized polymer resin serves as a crystal nucleating agent). Thesemethods may be employed individually or in combination.

With respect to the method in which a high volume ratio of very smalldiameter spherulites is achieved using a crystal nucleating agent, it ispreferred that a crystal nucleating agent is added to an oxymethylenepolymer resin in an amount of from 1 ppm by weight to 5% by weight, moreadvantageously from 5 ppm by weight to 3% by weight, based on the weightof the oxymethylene polymer resin. Preferred examples of crystalnucleating agents used in the present invention include boron nitride,talc, silica, mica and carbon black. Alternatively, a high volume ratioof very small diameter spherulites can also be achieved by a method inwhich at least one polymer resin selected from the group consisting ofan oxymethylene polymer resin having a low degree of polymerization (forexample, one having a melt index of more than 50 g/10 min) (referred tosimply as "LDP polymer") and an oxymethylene polymer resin having abranched or a crosslinked structure (referred to simply as"branched/crosslinked polymer") is used in combination with anon-branched and non-crosslinked oxymethylene polymer resin having amelt index of 1 to 50 g/10 min, wherein at least one polymer resin whichis selected from the group consisting of the LDP polymer and thebranched/crosslinked polymer is used in an amount of 0.1 to 50% byweight, based on the total weight of at least one polymer resin and thenon-branched and non-crosslinked polymer resin.

An explanation is made below on the method in which a high volume ratioof very small diameter spherulites is achieved by utilizing improvedconditions for the plasticization of the oxymethylene polymer resin inorder to cause the self-nucleating phenomenon. The method comprises anoperation in which, prior to the molding, the oxymethylene polymer resinis plasticized at a temperature in the range of from the meltingtemperature of the oxymethylene polymer resin to 10° C. above themelting temperature, more advantageously a temperature in the range offrom the melting temperature of the oxymethylene polymer resin to 8° C.above the melting temperature.

An oxymethylene homopolymer resin has a melting temperature of 176° C.Conventionally, prior to the molding of such polyoxymethylene polymerresin (having a melting temperature of 176° C.), the plasticizationthereof is generally performed at 200 to 220° C. However, according tothe above method utilizing the improved plasticization conditionsdefined in the present invention, prior to the molding, suchoxymethylene polymer resin (having a melting temperature of 176° C.) isplasticized at 176 to 186° C., more advantageously 176 to 184° C.

The effect of the self-nucleating phenomenon for achieving a high volumeratio of very small diameter spherulites can be largely increased byemploying an appropriate oxymethylene polymer resin. Specifically, whenthe oxymethylene polymer resin to be subjected to the molding has notbeen heated to a temperature that is 15° C., or more higher than themelting temperature of the oxymethylene polymer resin, not only can theformation of very small diameter spherulites be further facilitated, butalso the shaped article exhibits further excellent mechanicalproperties. The reason why such excellent effects can be obtained whenthe oxymethylene polymer resin to be subjected to the molding has lessthermal history as mentioned above is presumed to be due to the factthat the use of an oxymethylene polymer resin having a highcrystallinity or exhibiting a low degree of molecular chain entanglementadvantageous affects the crystallization behavior of the polymer resinunder the molding conditions employed in the present invention.

It is preferred that the oxymethylene polymer resin shaped article ofthe present invention is produced by injection molding or extrusionmolding. For the oxymethylene polymer resin shaped article to exhibit asatisfactorily improved flexural modulus, it is important that theoxymethylene polymer resin shaped article be uniformly crystallizedthroughout the interior thereof. When the thickness of the shapedarticle is less than 1 mm, it is difficult to achieve a uniformcrystallization for the oxymethylene polymer resin shaped article. Thereason for this is as follows. Generally, an oxymethylene polymer resinshaped article has an internal structure wherein a crystallized resinlayer is sandwiched between two outermost skin layers each having amolecular orientation. When the thickness of the shaped article is lessthan 1 mm, the ratio of the above-mentioned two skin layers in theshaped article is very large, so that the ratio of the crystallizedresin layer sandwiched between the skin layers inevitably becomes verysmall. In such a case, it becomes difficult to control the cooling rateof the resin during the crystallization thereof, so that a non-uniformcrystallization is likely occur in the shaped article. To avoid thisproblem, it is required that the thickness of the shaped article be atleast 1 mm, as measured even at a portion having a minimum thickness.

If desired, additives may be incorporated into the oxymethylene polymerresin to be used for producing the shaped article of the presentinvention as long as the desired properties of the shaped article of thepresent invention are not sacrificed. The addition of additives to theresin and a kneading of the resultant mixture can be conducted before orduring the molding. Examples of additives include polymeric materialsother than the oxymethylene polymer resin used as a molding material forproducing the shaped article; a multiphase interpolymer; variousstabilizers, such as an antioxidant, a heat stabilizer and anultraviolet light absorber; a dye; a pigment; a fibrous reinforcement; aparticulate reinforcement; a flame retardant; and molding propertymodifiers, such as a plasticizer and a mold release agent.

The oxymethylene polymer resin shaped article of the present inventionhas an improved crystal structure, and exhibits remarkably improvedmechanical properties even when containing very little or noreinforcement (filler). Therefore, the shaped article of the presentinvention can be advantageously used as a material for a sliding partand a precision part which are generally formed from an oxymethylenepolymer resin. Specific examples of such sliding parts and precisionparts include a gear, a bearing, a lever, a key stem, a cam, a ratchet,a roller, a screw, toy parts, a pipe, a fan, and a precision partcomprised of a composite material. Further, the shaped article of thepresent invention is advantageous not only in that it exhibits highflexural modulus at a high temperature, but also in that it exhibitsexcellent heat conductivity by virtue of a high crystallinity.Therefore, the shaped article of the present invention can beadvantageously used as a material for electric and electronic parts,such as a connector, a socket, a switch, a dial and a pin.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, the present invention will be described in more detail withreference to the following Examples and Comparative Examples, whichshould not be construed as limiting the scope of the present invention.

In the Examples and Comparative Examples, various properties of shapedarticles were evaluated by the following methods.

(1) Melting Temperature

The melting temperature was measured using a differential scanningcalorimetry apparatus (Model DSC7, manufactured and sold by Perkin ElmerCetus Co., Ltd., U.S.A.). In the measurement, use is made of a sampleprepared by a method in which an oxymethylene polymer resin was moldedinto a film by means of a press molding machine heated at 200° C. andthe obtained film was cut so as to obtain a sample having a weight of 5mg. The measurement was conducted under changing temperature conditionssuch that the temperature of the atmosphere surrounding the sample waselevated from 30° C. to 200° C. at a rate of 320° C./min and maintainedat 200° C. for 2 minutes, whereupon the temperature was lowered to 130°C. at a rate of 10° C./min, and finally, elevated from 130° C. at a rateof 2.5° C./min. During the final temperature elevation, a heatabsorption of the sample due to the melting of the sample was monitored,and the temperature corresponding to the apex of a peak of the heatabsorption was taken as the melting temperature of the sample.

(2) Crystallinity

An oxymethylene polymer resin shaped article (the size of which ismentioned below) was subjected to cutting to reduce the width and lengththereof while maintaining the thickness thereof so as to obtain a squaresample (of the same thickness as that of the shaped article) having asize of 10 mm×10 mm. Using a wide-angle X-ray diffraction apparatus(manufactured and sold by MAC Science Co., Ltd., Japan), the squaresample was subjected to an X-ray diffraction analysis, in which asurface of the sample perpendicular to the thicknesswise direction ofthe sample was scanned in a widthwise direction of the original shapedarticle while measuring the X-ray diffraction intensity by the 2 θ/θscanning method under measuring conditions wherein the radiation sourcewas Cu, the tube voltage was 40 kV and the tube current was 180 mA. Thecrystallinity of the sample was evaluated by means of a calculationmodule utilizing the peak-area crystallinity method (manufactured andsold by MAC Science Co., Ltd., Japan). Specifically, the crystallinity(Xc) was determined using the below-mentioned formula 1, from the areasof peaks at diffraction angles (2 θ) of around 22.9° and around 34.6°,which are, respectively, ascribed to the (100) crystal face diffractionand the (105) crystal face diffraction of the oxymethylene polymerresin, and from the area of a halo at a diffraction angle (2 θ) ofaround 20.0°. ##EQU1## (3) Crystallite Size

An oxymethylene polymer resin shaped article (the size of which ismentioned below) was subjected to cutting to reduce the width and lengththereof while maintaining the thickness thereof so as to obtain a squaresample (of the same thickness as that of the shaped article) having asize of 10 mm×10 mm. Using a wide-angle X-ray diffraction apparatus(manufactured and sold by MAC Science Co., Ltd., Japan), the squaresample was subjected to an X-ray diffraction analysis, in which asurface of the sample perpendicular to the thicknesswise direction ofthe sample was scanned in a widthwise direction of the original shapedarticle while measuring the X-ray diffraction intensity by the 2 θ/θscanning method under measuring conditions wherein the radiation sourcewas Cu, the tube voltage was 40 kV and the tube current was 180 mA. Thecrystallite size of the sample was evaluated by means of a calculationmodule utilizing the Sherrer's method (manufactured and sold by MACScience Co., Ltd., Japan), wherein the correction of the calculation wasconducted using a calibration curve obtained from diffraction analysisdata of a silicon powder. The calculation was performed by a method inwhich the value of a half width of the peak at a diffraction angle (2 θ)of around 34.6° C., ascribed to the (105) crystal face diffraction ofthe oxymethylene polymer resin, was obtained, and the obtained value ofthe half width was applied to the Sherrer's equation, therebydetermining the crystallite size of the oxymethylene polymer resin.

(4) Spherulite Diameter and Volume Percentage

An oxymethylene polymer resin shaped article was cut by means of amicrotome along a plane perpendicular to a surface thereof, therebyexposing a new surface. From the exposed new surface of the shapedarticle, a slice (specimen) having a thickness of about 5 mm wasobtained using a microtome. The obtained specimen was observed through apolarization microscope (model Nikon-PFL, manufactured and sold by NikonCorp., Japan) at a 400-time magnification in order to measure thediameters of the spherulites in the observed surface of the specimen.The volume ratio of spherulites having a specific diameter or more tothe whole volume of the shaped article was calculated from the resultantdata of the diameters of the spherulites in the observed surface of thespecimen. Generally, the spherulite diameter of an oxymethylene polymerresin shaped article exhibits a relatively broad distribution and variesin accordance with the distance from the surface of the shaped article,and, therefore, spherulites each having substantially the same diameterare present at the same distance from the surface of the shaped article.By contrast, the production conditions employed in the present inventionare advantageous not only in that a high volume ratio of very smalldiameter spherulites can be achieved, but also in that the spherulitediameter of the shaped article exhibits an extremely narrow distributionthroughout the interior of the shaped article except for the outermostskin layers, that is, substantially all spherulites in the shapedarticle have substantially the same diameter, irrespective of thedistance from the surface of the shaped article. Therefore, by measuringthe area percentage of spherulites having a specific diameter in theentire area of the observed surface of the above specimen of the shapedarticle, the volume percentage of such spherulites having the specificdiameter in the whole volume of the shaped article can be determined.

(5) Flexural Modulus

The flexural modulus of an oxymethylene polymer resin shaped article wasevaluated at a testing environmental temperature of 23° C. in accordancewith ASTM D-790. In the present invention, among various propertiesusable as an index of the mechanical strength of a shaped article, aflexural modulus is employed. The reason for the selection of a flexuralmodulus as an index is because the flexural modulus clearly representsthe stiffness of a shaped article, which is a practically importantproperty.

In the Examples and Comparative Examples, the below-mentionedoxymethylene polymer resins were subjected to molding. The measurementof the melt index (g/10 min) of the oxymethylene polymer resins wasconducted in accordance with ASTM D-1238-57T (conditions E).

(1) POM1: Oxymethylene Homopolymer

POM1 was produced by a method in which a formaldehyde polymer having amelt index of 9.9 g/10 min and having its terminals stabilized by amethoxy group and an acetyl group is blended with 5 ppm by weight of aboron nitride powder (GP, average particle diameter: 3.5 μm,manufactured and sold by Denki Kagaku Kogyo Kabushiki Kaisha, Japan) asa nucleating agent, and the resultant blend is subjected to kneading andpelletization by means of an extruder which is set at a temperature of200° C., thereby obtaining pellets (melting temperature of POM1: 176°C.).

(2) POM2: Oxymethylene Homopolymer

POM2 was produced by a method in which a formaldehyde polymer having amelt index of 9.9 g/10 min and having its terminals stabilized by amethoxy group and an acetyl group is blended with 10 ppm by weight of aboron nitride powder (GP, average particle diameter: 3.5 μm,manufactured and sold by Denki Kagaku Kogyo Kabushiki Kaisha, Japan) asa nucleating agent, and the resultant blend is subjected to kneading andpelletization by means of an extruder which is set at a temperature of200° C., thereby obtaining pellets (melting temperature of POM2: 177°C.).

(3) POM3: Oxymethylene Homopolymer

POM3 was produced by a method in which a formaldehyde polymer having amelt index of 9.9 g/10 min and having its terminals stabilized by amethoxy group and an acetyl group is blended with 20 ppm by weight of aboron nitride powder (GP, average particle diameter: 3.5 μm,manufactured and sold by Denki Kagaku Kogyo Kabushiki Kaisha, Japan) asa nucleating agent, and the resultant blend is subjected to kneading andpelletization by means of an extruder which is set at a temperature of200° C., thereby obtaining pellets (melting temperature of POM3: 177°C.).

(4) POM4: Oxymethylene Homopolymer

POM4 was produced by a method in which a formaldehyde polymer having amelt index of 9.9 g/10 min and having its terminals stabilized by amethoxy group and an acetyl group is subjected to kneading andpelletization by means of an extruder which is set at a temperature of200° C., thereby obtaining pellets (melting temperature of POM4: 176°C.).

(5) POM5: Oxymethylene Homopolymer

POM5 was a powdery formaldehyde polymer having a particle diameter ofseveral tens of μm. POM5 had a melt index of 9.9 g/10 min and had itsterminals stabilized by a methoxy group and an acetyl group. POM5 hadnot been heated to a temperature of 190° C. or more after the productionthereof (melting temperature of POM5: 177° C.).

(6) POM6: Oxymethylene Copolymer

POM6 was produced by the following method. Trioxane was copolymerizedwith ethylene oxide as a comonomer to obtain an oxymethylene copolymerhaving a melt index of 9.0 g/10 min and having an ethylene oxide unitcontent of 1.0 mole, relative to 100 moles of the oxymethylene units.The obtained oxymethylene copolymer was blended with 5 ppm by weight ofa boron nitride powder (GP, average particle diameter: 3.5 μm,manufactured and sold by Denki Kagaku Kogyo Kabushiki Kaisha, Japan) asa nucleating agent, and the resultant blend was subjected to kneadingand pelletization by means of an extruder which was set at a temperatureof 200° C., thereby obtaining pellets (melting temperature of POM6: 166°C.).

(7) POM7: Oxymethylene Copolymer

POM7 was produced by the following method. Trioxane was copolymerizedwith ethylene oxide as a comonomer to obtain an oxymethylene copolymerhaving a melt index of 9.0 g/10 min and having an ethylene oxide unitcontent of 1.0 mole, relative to 100 moles of the oxymethylene units.The obtained oxymethylene copolymer was subjected to kneading andpelletization by means of an extruder which was set at a temperature of200° C., thereby obtaining pellets (melting temperature of POM7: 166°C.).

(8) POM8: Oxymethylene Copolymer

POM8 was produced by the following method. Trioxane was copolymerizedwith ethylene oxide as a comonomer to obtain an oxymethylene copolymerhaving a melt index of 9.0 g/10 min and having an ethylene oxide unitcontent of 1.6 moles, relative to 100 moles of the oxymethylene units.The obtained oxymethylene copolymer was blended with 5 ppm by weight ofa boron nitride powder (GP, average particle diameter: 3.5 μm,manufactured and sold by Denki Kagaku Kogyo Kabushiki Kaisha, Japan) asa nucleating agent, and the resultant blend was subjected to kneadingand pelletization by means of an extruder which was set at a temperatureof 200° C., thereby obtaining pellets (melting temperature of POM8: 164°C.).

(9) POM9: Oxymethylene Copolymer

POM9 was produced by the following method. Trioxane was copolymerizedwith ethylene oxide as a comonomer to obtain an oxymethylene copolymerhaving a melt index of 9.0 g/10 min and having an ethylene oxide unitcontent of 1.6 moles, relative to 100 moles of the oxymethylene units.The obtained oxymethylene copolymer was subjected to kneading andpelletization by means of an extruder which was set at a temperature of200° C., thereby obtaining pellets (melting temperature of POM9: 164°C.).

The molding methods employed in the Examples and Comparative Examplesare as follows.

(a) Conditions for plasticization: An oxymethylene polymer resin wasplasticized by means of a molding machine (for injection molding orextrusion molding) which was set at a specific cylinder temperature or ahot press molding machine (press machine) which was heated at a specifictemperature.

With respect to the molding machines used for plasticization, thefollowing molding machines are identified.

Molding machine 1 (injection): the cylinder temperature was 200° C.

Molding machine 2 (injection): the cylinder temperature was 186° C.

Molding machine 3 (injection): the cylinder temperature was 184° C.

Molding machine 4 (injection): the cylinder temperature was 182° C.

Molding machine 5 (injection): the cylinder temperature was 176° C.

Molding machine 6 (injection): the cylinder temperature was 174° C.

Molding machine 7 (extrusion): the cylinder temperature was 200° C.

Molding machine 8 (extrusion): the cylinder temperature was 186° C.

Molding machine 9 (extrusion): the cylinder temperature was 184° C.

Press machine 1: the press temperature was 200° C.

Press machine 2: the press temperature was 182° C.

Press machine 3: the press temperature was 180° C.

(b) Molding Conditions Injection molding: From a polymer resin, a testpiece for a flexural test was prepared in accordance with ASTM D-790,using an injection molding machine which was set at a specific cylindertemperature.

With respect to the conditions used for injection molding, the followingconditions are identified.

Injection molding 1: the mold temperature was 146° C. and the coolingtime was 30 seconds.

Injection molding 2: the mold temperature was 151° C. and the coolingtime was 120 seconds.

Injection molding 3: the mold temperature was 153° C. and the coolingtime was 180 seconds.

Injection molding 4: the mold temperature was 141° C. and the coolingtime was 30 seconds.

Injection molding 5: the mold temperature was 70° C. and the coolingtime was 30 seconds. Extrusion molding: From a polymer resin, a testpiece was prepared using an extrusion molding machine which was set at aspecific cylinder temperature and using a flat sheeting die having athickness of 5 mm.

With respect to the conditions used for extrusion molding, the followingconditions are identified.

Extrusion molding 1: the mold temperature was 146° C.

Extrusion molding 2: the mold temperature was 153° C.

Extrusion molding 3: the mold temperature was 30° C. Press molding(compression molding): A mold filled with a polymer resin was subjectedto hot pressing under a pressure of 100 kg/cm² by means of a hot pressmolding machine which was heated at a predetermined temperature, therebymolding the polymer resin, and then the resultant molded product wascooled at a specific cooling rate by controlling the temperature of thepress molding machine, to thereby obtain a test piece.

With respect to the conditions used for press molding, the followingconditions are identified.

Press molding 1: the plasticized resin in the mold was cooled to 70° C.at a cooling rate of 50° C./min.

Press molding 2: the plasticized resin in the mold was cooled to 70° C.at a cooling rate of 5° C./min.

Press molding 3: the plasticized resin in the mold was cooled to 150° C.at a cooling rate of 2° C./min, and then maintained at 150° C. for 3hours.

Press molding 4: the plasticized resin in the mold was cooled to 153° C.at a cooling rate of 2° C./min, and then maintained at 153° C. for 3hours.

(c) Shape of a sample (test piece) of shaped article: A sample of shapedarticle had the following size: a thickness of about 3 mm, about 1 mm orabout 0.5 mm; a width of about 13 mm; and a length of about 130 mm.

(d) Annealing conditions: A molded test piece was treated in an air ovenwhich was set at a predetermined temperature for 1 hour.

With respect to the annealing conditions, the following conditions areidentified.

Annealing 1: no annealing.

Annealing 2: annealing temperature was 172° C.

Annealing 3: annealing temperature was 170° C.

Annealing 4: annealing temperature was 150° C.

Examples 1 to 11

Oxymethylene homopolymer resins were individually molded by using aninjection molding machine to thereby obtain test pieces (shapedarticles). The obtained test pieces were evaluated by conducting anobservation using a polarization microscope, a measurement of thecrystallinity and the average crystallite size, and a flexural test. Theconditions for the production of the test pieces and the results of theevaluation of the test pieces are shown in Table 1.

Examples 12 to 15

Oxymethylene homopolymer resins were individually molded by using anextrusion molding machine to thereby obtain test pieces (shapedarticles). The obtained test pieces were evaluated by conducting anobservation using a polarization microscope, a measurement of thecrystallinity and the average crystallite size, and a flexural test. Theconditions for the production of the test pieces and the results of theevaluation of the test pieces are shown in Table 1.

Examples 16 to 24

Oxymethylene homopolymer resins were individually molded by using a hotpress molding machine to thereby obtain test pieces (shaped articles)The obtained test pieces were evaluated by conducting an observationusing a polarization microscope, a measurement of the crystallinity andthe average crystallite size, and a flexural test. The conditions forthe production of the test pieces and the results of the evaluation ofthe test pieces are shown in Table 1.

Examples 25 to 28

Oxymethylene homopolymer resins were individually molded by using aninjection molding machine to thereby obtain test pieces (shapedarticles), and the obtained test pieces were subjected to annealing bymeans of an air oven. The resultant annealed test pieces were evaluatedby conducting an observation using a polarization microscope, ameasurement of the crystallinity and the average crystallite size, and aflexural test. The conditions for the production of the test pieces andthe results of the evaluation of the test pieces are shown in Table 1.With respect to the shaped article obtained in Example 26, the stiffnessthereof at a high temperature was evaluated by measuring its deflectiontemperature under load in accordance with ASTM-D648 (wherein atemperature at which a test piece is deflected by 0.2 mm under a load of18.6 kg/cm² is measured). It was found that the shaped article obtainedin Example 26 exhibited a deflection temperature under load of 155° C.

Comparative Examples 1 to 6

Oxymethylene homopolymer resins were individually molded by using aninjection molding machine to thereby obtain test pieces (shapedarticles). The obtained test pieces were evaluated by conducting anobservation using a polarization microscope, a measurement of thecrystallinity and the average crystallite size, and a flexural test. Theconditions for the production of the test pieces and the results of theevaluation of the test pieces are shown in Table 1. With respect to theshaped article obtained in Comparative Example 1, the stiffness thereofat a high temperature was evaluated by measuring its deflectiontemperature under load in accordance with ASTM-D648 (wherein atemperature at which a test piece is deflected by 0.2 mm under a load of18.6 kg/cm² is measured). It was found that the shaped article obtainedin Comparative Example 1 exhibited a deflection temperature under loadof 136° C.

Comparative Example 7

An oxymethylene homopolymer resin was molded by using an extrusionmolding machine to thereby obtain a test piece (shaped article). Theobtained test piece was evaluated by conducting an observation using apolarization microscope, a measurement of the crystallinity and theaverage crystallite size, and a flexural test. The conditions for theproduction of the test piece and the results of the evaluation of thetest piece are shown in Table 1.

Examples 29 to 30

Oxymethylene copolymer resins were individually molded by using aninjection molding machine to thereby obtain test pieces (shapedarticles). The obtained test pieces were evaluated by conducting anobservation using a polarization microscope, a measurement of thecrystallinity and the average crystallite size, and a flexural test. Theconditions for the production of the test pieces and the results of theevaluation of the test pieces are shown in Table 2.

Comparative Examples 8 to 13

Oxymethylene copolymer resins were individually molded by using aninjection molding machine to thereby obtain test pieces (shapedarticles). The obtained test pieces were evaluated by conducting anobservation using a polarization microscope, a measurement of thecrystallinity and the average crystallite size, and a flexural test. Theconditions for the production of the test pieces and the results of theevaluation of the test pieces are shown in Table 2.

Comparative Example 14

An oxymethylene copolymer resin was molded by using an injection moldingmachine to thereby obtain a test piece (shaped article), and theobtained test piece was subjected to annealing by means of an air oven.The resultant annealed test piece was evaluated by conducting anobservation using a polarization microscope, a measurement of thecrystallinity and the average crystallite size, and a flexural test. Theconditions for the production of the test piece and the results of theevaluation of the test piece are shown in Table 2.

                                      TABLE 1                                     __________________________________________________________________________                                              Diameter of                         Molding                                   spherulites                         material                              Average                                                                           constituting                        (oxymethy-                            crystal-                                                                          70% by volume                       lene         Plastici-        Thick-                                                                            Crystal-                                                                          lite                                                                              or more of the                                                                          Flexural                  homopolymer  zation                                                                              Molding                                                                            Annealing                                                                           ness                                                                              linity                                                                            size                                                                              volume of the                                                                           modulus                   resin)       conditions*)                                                                        conditions                                                                         conditions                                                                          (mm)                                                                              (%) (Å)                                                                           article (μm)                                                                         (GPa)                     __________________________________________________________________________    Example 1                                                                           POM1   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   75  152 30        3.8                                    machine 1                                                                           molding 1                                                  Example 2                                                                           POM1   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   78  164 35        3.9                                    machine 1                                                                           molding 2                                                  Example 3                                                                           POM2   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   78  165 15        4                                      machine 1                                                                           molding 2                                                  Example 4                                                                           POM3   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   80  180 10        4.8                                    machine 1                                                                           molding 3                                                  Example 5                                                                           POM4   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   76  155 28        3.9                                    machine 2                                                                           molding 1                                                  Example 6                                                                           POM4   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   77  158 15        4                                      machine 3                                                                           molding 1                                                  Example 7                                                                           POM4   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   78  166 15        4.2                                    machine 3                                                                           molding 2                                                  Example 8                                                                           POM4   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   80  172 15        4.8                                    machine 3                                                                           molding 3                                                  Example 9                                                                           POM5   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   79  168  3        4.6                                    machine 3                                                                           molding 2                                                  Example 10                                                                          POM5   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   81  190  3        5.3                                    machine 4                                                                           molding 3                                                  Example 11                                                                          POM1   Molding                                                                             Injection                                                                          Annealing 1                                                                         1   74  152 30        3.6                                    machine 1                                                                           molding 1                                                  Example 12                                                                          POM1   Molding                                                                             Extrusion                                                                          Annealing 1                                                                         3   76  153 30        3.9                                    machine 7                                                                           molding 1                                                  Example 13                                                                          POM3   Molding                                                                             Extrusion                                                                          Annealing 1                                                                         3   82  181 10        4.9                                    machine 7                                                                           molding 2                                                  Example 14                                                                          POM4   Molding                                                                             Extrusion                                                                          Annealing 1                                                                         3   77  156 28        4.0                                    machine 8                                                                           molding 1                                                  Example 15                                                                          POM4   Molding                                                                             Extrusion                                                                          Annealing 1                                                                         3   81  175 15        4.9                                    machine 9                                                                           molding 2                                                  Example 16                                                                          POM2   Press Press                                                                              Annealing 1                                                                         3   76  166 20        3.9                                    machine 1                                                                           molding 1                                                  Example 17                                                                          POM2   Press Press                                                                              Annealing 1                                                                         3   78  168 22        4.1                                    machine 1                                                                           molding 2                                                  Example 18                                                                          POM2   Press Press                                                                              Annealing 1                                                                         3   79  172 22        4.3                                    machine 1                                                                           molding 3                                                  Example 19                                                                          POM3   Press Press                                                                              Annealing 1                                                                         3   81  190 15        4.9                                    machine 1                                                                           molding 4                                                  Example 20                                                                          POM4   Press Press                                                                              Annealing 1                                                                         3   77  167 13        4.2                                    machine 2                                                                           molding 1                                                  Example 21                                                                          POM4   Press Press                                                                              Annealing 1                                                                         3   78  169 14        4.5                                    machine 2                                                                           molding 2                                                  Example 22                                                                          POM4   Press Press                                                                              Annealing 1                                                                         3   79  175 13        4.7                                    machine 2                                                                           molding 3                                                  Example 23                                                                          POM5   Press Press                                                                              Annealing 1                                                                         3   79  180  3        5.2                                    machine 2                                                                           molding 3                                                  Example 24                                                                          POM5   Press Press                                                                              Annealing 1                                                                         3   81  220  3        6.3                                    machine 3                                                                           molding 4                                                  Example 25                                                                          POM2   Molding                                                                             Injection                                                                          Annealing 3                                                                         3   79  170 15        4.4                                    machine 1                                                                           molding 5                                                  Example 26                                                                          POM3   Molding                                                                             Injection                                                                          Annealing 2                                                                         3   80  200 10        5.2                                    machine 1                                                                           molding 5                                                  Example 27                                                                          POM4   Molding                                                                             Injection                                                                          Annealing 3                                                                         3   79  175 15        4.6                                    machine 3                                                                           molding 5                                                  Example 28                                                                          POM5   Molding                                                                             Injection                                                                          Annealing 2                                                                         3   80  200  3        6.2                                    machine 3                                                                           molding 5                                                  Comparative                                                                         POM2   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   69  110 15        3.2                       Example 1    machine 1                                                                           molding 5                                                  Comparative                                                                         POM4   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   69  106 120       3.2                       Example 2    machine 1                                                                           molding 5                                                  Comparative                                                                         POM4   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   76  164 120       3.4                       Example 3    machine 1                                                                           molding 1                                                  Comparative                                                                         POM4   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   78  164 120       3.4                       Example 4    machine 1                                                                           molding 2                                                  Comparative                                                                         POM4   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   69  110 15        3.2                       Example 5    machine 3                                                                           molding 5                                                  Comparative                                                                         POM1   Molding                                                                             Injection                                                                          Annealing 1                                                                         0.5 72  150 30        3.3                       Example 6    machine 1                                                                           molding 1                                                  Comparative                                                                         POM2   Molding                                                                             Extrusion                                                                          Annealing 1                                                                         3   68  105 15        3.2                       Example 7    machine 7                                                                           molding 3                                                  __________________________________________________________________________     *Note:                                                                        Molding machines 1 to 6 are injection molding machines.                       Molding machines 7 to 9 are extrusion molding machines.                       Press machines 1 to 3 are press molding machines.                        

                                      TABLE 2                                     __________________________________________________________________________                                              Diameter of                         Molding                                   spherulites                         material                              Average                                                                           constituting                        (oxymethy-                            crystal-                                                                          70% by volume                       lene         Plastici-        Thick-                                                                            Crystal-                                                                          lite                                                                              or more of the                                                                          Flexural                  homopolymer  zation                                                                              Molding                                                                            Annealing                                                                           ness                                                                              linity                                                                            size                                                                              volume of the                                                                           modulus                   resin)       conditions*)                                                                        conditions                                                                         conditions                                                                          (mm)                                                                              (%) (Å)                                                                           article (μm)                                                                         (GPa)                     __________________________________________________________________________    Example 29                                                                          POM6   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   72  160 45        3                                      machine 1                                                                           molding 4                                                  Example 30                                                                          POM7   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   72  161 30        3                                      machine 5                                                                           molding 4                                                  Comparative                                                                         POM6   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   65  112 45        2.6                       Example 8    machine 1                                                                           molding 5                                                  Comparative                                                                         POM7   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   65  112 140       2.6                       Example 9    machine 1                                                                           molding 5                                                  Comparative                                                                         POM7   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   67  150 140       2.7                       Example 10   machine 1                                                                           molding 4                                                  Comparative                                                                         POM7   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   65  112 30        2.6                       Example 11   machine 5                                                                           molding 5                                                  Comparative                                                                         POM8   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   62  110 40        2.5                       Example 12   machine 1                                                                           molding 4                                                  Comparative                                                                         POM9   Molding                                                                             Injection                                                                          Annealing 1                                                                         3   62  110 31        2.5                       Example 13   machine 6                                                                           molding 4                                                  Comparative                                                                         POM6   Molding                                                                             Injection                                                                          Annealing 4                                                                         3   67  150 45        2.7                       Example 14   machine 1                                                                           molding 5                                                  __________________________________________________________________________     *Note:                                                                        Molding machines 1 to 6 are injection molding machines.                       Molding machines 7 to 9 are extrusion molding machines.                       Press machines 1 to 3 are press molding machines.                        

INDUSTRIAL APPLICABILITY

The oxymethylene polymer resin shaped article of the present inventionhas an improved crystal structure, and exhibits remarkably improvedmechanical properties even when containing very little or noreinforcement (filler). Therefore, the shaped article of the presentinvention can be advantageously used as a material for a sliding partand a precision part which are generally formed from an oxymethylenepolymer resin. Specific examples of such sliding parts and precisionparts include a gear, a bearing, a lever, a key stem, a cam, a ratchet,a roller, a screw, toy parts, a pipe, a fan, and a precision partcomprised of a composite material. Further, the shaped article of thepresent invention is advantageous not only in that it exhibits highflexural modulus at a high temperature, but also in that it exhibitsexcellent heat conductivity by virtue of the high crystallinity.Therefore, the shaped article of the present invention can beadvantageously used as a material for electric and electronic parts,such as a connector, a socket, a switch, a dial and a pin.

What is claimed is:
 1. An oxymethylene polymer resin shaped articlehaving high flexural modulus, which is produced by molding anoxymethylene polymer resin, said oxymethylene polymer resin beingselected from the group consisting of an oxymethylene homopolymer resin,an oxymethylene copolymer resin and a mixture thereof, said oxymethylenecopolymer resin comprising polymer chains comprised of oxymethylenemonomer units and oxyalkylene comonomer units, wherein said oxyalkylenecomonomer units are randomly present in said polymer chains collectivelyin an amount of from 0.01 to 1.0 mole, relative to 100 moles of saidoxymethylene monomer units,said shaped article having the followingcharacteristics (1) to (4):(1) a crystallinity of 72% or more; (2) anaverage crystallite size of 150 Å or more; (3) 70% by volume or more ofthe whole volume of said shaped article being comprised of spheruliteseach having a diameter of 60 μm or less; and (4) a thickness of 1 mm ormore.
 2. The shaped article according to claim 1, wherein the molding isperformed by injection or extrusion.
 3. The shaped article according toclaim 1 or 2, wherein, in said characteristic (3), 70% by volume or moreof the whole volume of said shaped article is comprised of spheruliteseach having a diameter of 30 μm or less.
 4. The shaped article accordingto claim 1 or 2, wherein said oxymethylene polymer resin contains acrystal nucleating agent in an amount of from 1 ppm by weight to 5% byweight.
 5. The shaped article according to claim 4, wherein said crystalnucleating agent is at least one compound selected from the groupconsisting of boron nitride, talc, silica, mica and carbon black.
 6. Theshaped article according to claim 1 or 5, wherein, prior to the molding,said oxymethylene polymer resin is plasticized at a temperature in therange of from the melting temperature of said resin to 10° C. above saidmelting temperature.
 7. The shaped article according claim 1 to 2,wherein, prior to the molding, said oxymethylene polymer resin has notbeen heated to a temperature that is 15° C. or more higher than themelting temperature of said resin.
 8. The shaped article according toclaim 1 to 2, wherein said oxymethylene polymer resin is crystallized ata temperature in the range of from 30° C. below the melting temperatureof said resin to said melting temperature during or after the molding.9. The shaped article according to claim 1 to 2, wherein saidoxymethylene polymer resin is annealed at a temperature in the range offrom 10° C. below the melting temperature of said resin to said meltingtemperature after the molding.
 10. The shaped article according to claim1 to 2, wherein said oxymethylene polymer resin is an oxymethylenehomopolymer resin.
 11. The shaped article according to claim 10, whichhas a flexural modulus of 3.8 GPa or more.
 12. The shaped articleaccording to claim 1 to 2, which is a part for an office automationmachine, a part for an electric apparatus, a part for electronicapparatus or a part for an automobile.
 13. An oxymethylene polymer resinshaped article having high flexural modulus, which is produced bymolding an oxymethylene polymer resin, said oxymethylene polymer resinbeing selected from the group consisting of an oxymethylene homopolymerresin, an oxymethylene copolymer resin and a mixture thereof, saidoxymethylene copolymer resin comprising polymer chains comprised ofoxymethylene monomer units and oxyalkylene comonomer units, wherein saidoxyalkylene comonomer units are randomly present in said polymer chainscollectively in an amount of from 0.01 to 1.0 mole, relative to 100moles of said oxymethylene monomer units, and wherein said oxymethylenepolymer resin contains a crystal nucleating agent in an amount of from 1ppm by weight to 5% by weight;said shaped article having the followingcharacteristics (1) to (4):(1) a crystallinity of 72% or more; (2) anaverage crystallite size of 150 Å or more; (3) 70% by volume or more ofthe whole volume of the shaped article being comprised of spheruliteseach having a diameter of 60 μm or less; and (4) a thickness of 1 mm ormore.
 14. The shaped article according to claim 13, wherein said crystalnucleating agent is at least one compound selected from the groupconsisting of boron nitride, talc, silica, mica and carbon black.
 15. Anoxymethylene polymer resin shaped article having high flexural modulus,which is produced by molding an oxymethylene polymer resin, saidoxymethylene polymer resin being selected from the group consisting ofan oxymethylene homopolymer resin, an oxymethylene copolymer resin and amixture thereof, said oxymethylene copolymer resin comprising polymerchains comprised of oxymethylene monomer units and oxyalkylene comonomerunits, wherein said oxyalkylene comonomer units are randomly present insaid polymer chains collectively in an amount of from 0.01 to 1.0 mole,relative to 100 moles of said oxymethylene monomer units; and whereinprior to molding, said oxymethylene polymer resin is plasticized at atemperature in the range of from the melting temperature of said resinto 10° C. above said melting temperature;said shaped article having thefollowing characteristics (1) to (4):(1) a crystallinity of 72% or more;(2) an average crystallite size of 150 Å or more; (3) 70% by volume ormore of the whole volume of the shaped article being comprised ofspherulites each having a diameter of 60 μm or less; and (4) a thicknessof 1 mm or more.